of Edinburgh, Session 1877 - 78 . 
735 
not only are the double sulphates of the didymium group some- 
what soluble in a saturated solution of potassic sulphate, but that 
some erbium, if not also some yttrium, is precipitated. Repetition 
of this process fails to remove the last traces of the didymium 
group. 
After trying various modifications of this method, some of which 
gave better results, notably boiling the double sulphates with the 
saturated potassic-sulpliate solution and filtering hot, we determined 
to look for another method, none of these variations being suffi- 
ciently good. 
We have obtained better results by the following method. 
After having extracted as much as possible of the didymium 
group by the old method, the earths are dissolved in nitric acid, 
and to this solution a large excess of a solution of carbonate of 
ammonia is added, and the whole allowed to digest for a day in a 
closed flask, the precipitate being frequently shaken up. The 
liquid is then filtered and the undissolved residue washed. On 
acidifying the filtrate with hydrochloric acid, and adding oxalic 
acid, a precipitate is obtained which, on ignition, yields the oxides 
of yttrium and erbium, free from, or only containing the merest 
trace of, lanthanum or didymium, which may be removed by a 
repetition of the process. Any cerium originally present goes 
into solution, and must be removed by boiling a solution of the 
sulphates with carbonate of magnesia. The undissolved residue of 
the carbonate of ammonia solution still contains yttrium and 
erbium carbonates, but is much richer in didymium, and must be 
treated with a fresh portion of carbonate of ammonia solution. 
It is essential that the ammonium carbonate solution be neither 
too concentrated or too dilute, as in the first case some lanthanum 
and didymium dissolves, and in the latter the carbonates of yttrium 
and erbium, after dissolving, crystallize out as double salts, leaving 
almost nothing in solution. The strength we found most suitable 
was about half saturated. 
